The present invention relates to a digital broadcast receiver for receiving a broadcast signal in which a plurality of subcarrier signals, each modulated by differential phase-shift keying, are multiplexed by orthogonal frequency-division multiplexing.
Systems for broadcasting audio signal data by orthogonal frequency-division multiplexing (referred to below as OFDM) have been standardized in recommendation BS.774 of the Radiotelecommunication Standardization Sector of the International Telecommunication Union (ITU-R). A data frame conforming to this recommendation begins with a zero-amplitude null symbol followed by a phase-reference symbol. In a conventional receiver, the signal received at the antenna is down-converted to an intermediate frequency by mixing with a signal generated by a local oscillator. The intermediate-frequency signal is detected by an orthogonal demodulator to produce a baseband signal, which is converted from analog to digital form, then processed by means of a discrete Fourier transform, thereby detecting subcarrier phase information. A differential demodulator takes the difference between the phase angles of successive symbols to obtain demodulated data, which are decoded to obtain an audio signal. The discrete Fourier transform processing is synchronized by means of an envelope detector that detects the null symbol at the beginning of each data frame. Both frame synchronization and symbol synchronization are controlled in this way.
The differential phase data output by the differential demodulator have nominal values of .pi./4, 3.pi./4, 5.pi./4, and 7.pi./4. Phase error is detected by multiplying the differential phase data by four, then dividing by 2.pi., producing remainders with nominal values of .pi.. The remainders are averaged over a certain number of symbols, then .pi. is subtracted from the average remainder to obtain an error value .epsilon., and the frequency of the local oscillator is tuned so as to reduce .epsilon. to zero.
This conventional method of tuning is inherently ambiguous, because .epsilon. is equal to zero not only when the phase error is zero, but also when the phase error is equal to .pi./2, .pi., or 3.pi./2. If the phase error is larger than .pi./4, the conventional method will usually tune the local oscillator to a frequency that produces a phase error of .pi./2, .pi., or 3.pi./2 instead of the desired phase error of zero. In other words, the conventional method cannot detect or correct phase errors equal to non-zero multiples of .pi./2.
A further deficiency of the conventional method is that the different subcarrier frequencies in the OFDM signal are not identified. If the local oscillator operates at a frequency that differs from the correct frequency by more than half the subcarrier frequency spacing, the conventional method will again tend to tune the local oscillator toward the incorrect frequency.
A still further problem arises from the use of envelop detection for frame and symbol synchronization. When the signal is contaminated with reflection or other noise, the envelop detector may be unable to detect the null symbol reliably and accurately, creating serious difficulties in synchronization. This problem is particularly troublesome when the receiver is mounted in a moving vehicle, the motion of which necessitates frequent fine adjustments of the synchronization timing.
The above problems are not limited to the reception of digital audio broadcasts conforming to ITU-R recommendation BS.774. Similar problems can occur with other digital broadcast signals of the same general type. These signals will be referred to generically as PSK-OFDM signals, PSK denoting phase-shift keying and OFDM denoting orthogonal frequency-division multiplexing. A PSK-OFDM signal will be assumed to have a frame synchronization symbol such as the above-mentioned null symbol, and a phase reference symbol that establishes a separate phase reference for each subcarrier, thereby enabling differential demodulation of the subsequent data symbol at each subcarrier frequency.